This invention relates to an aluminum evaporator core of the type used in an automotive air conditioning system. More particularly, this invention relates to such evaporator core comprising an upstream facade that includes a metallic copper deposit that reacts with condensate to generate cupric ions effective to inhibit biological growth within the core, but which deposit is electrically isolated from aluminum heat transfer surfaces to avoid accelerated corrosion thereof.
In a typical automotive air conditioning system, refrigerant is circulated through an evaporator core located at the front of the passenger compartment to cool air en route to the compartment. The evaporator core has a plate-and-fin construction formed of a stack of elements individually stamped from aluminum alloy sheet stock and brazed into an integral structure. Aluminum alloy is preferred because it has low specific gravity and high thermal transfer properties, and also because the evaporator may be fashioned from readily available sheet stock by convenient operations such as stamping and brazing.
The plate-and-fin design of the evaporator core provides numerous air passages for maximum contact with heat exchanger surfaces. During air conditioning operation, moisture tends to condense within the air passages of the evaporator core and promote the growth of bacteria or fungi, which, if excessive, may produce an unpleasant odor or otherwise affect passenger comfort. U.S. Ser. No. 360,681, filed by J. Siak et al, describes an aluminum evaporator core coated with a metallic copper plate to inhibit microbial growth. Condensate collecting in the air passages tends to be slightly acidic and reacts with the copper to produce cupric ions. The cupric ions, in turn, dissolve in the condensate and inhibit biological growth. The copper reaction with the condensate is slow. Further, only very low concentrations of cupric ions are effective to inhibit microbial growth. As a result, a thin plate may protect the evaporator for an extended time.
A potential corrosion problem arises where the substrate aluminum becomes exposed in physical and electrical contact with the copper plate. In the presence of an electrolyte such as the condensate, the exposed aluminum and the copper coating form a galvanic cell. This redox coupling enhances oxidation of the aluminum, which is evidenced by accelerated corrosion of the evaporator core. The evaporator described in the aforementioned patent application includes a zinc layer intermediate the copper plate and the aluminum substrate. In regions where the copper becomes eroded, the zinc layer provides a barrier to protect the underlying aluminum from contact with the condensate. Nevertheless, it is desirable to develop a system to deliver biocidal cupric ions generated by in-situ copper reaction while preventing a copper-aluminum galvanic coupling that would accelerate corrosion of the evaporator core.
Therefore, it is an object of this invention to provide an aluminum evaporator core for an automotive air conditioning system or the like which includes a copper plate for reaction with condensate to generate biocidal cupric ions, and further wherein the plate is physically and electrically isolated from the aluminum to avoid galvanic coupling of the type that would otherwise result in accelerated corrosion of the evaporator core, but is nevertheless strategically located to disperse the ions over evaporator core surfaces, including within air passages therein, so as to inhibit microbial growth.